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http://www.lanlogic.net/support/docs/article.aspx?id=10064
You may need to turn off cache mode and turn it back on if your Outlook
client is running slow, or if you are suddenly unable to send or receive
emails. Going through the steps will essentially remove the Outlook
cache from your computer, and then rebuild it. The cache is the local
copies of your emails.
1. Turn Off Cache Mode
1. Open Outlook
2. Tools
3. Email Accounts
4. Select View or change existing e-mail accounts
5. Next
6. With "Microsoft Exchange Server" selected, Click "Change"
7. Uncheck the box for "Use Cached Exchange Mode"
8. Next
9. In the box that pops up, click on OK
10. Click Finish
2. Close Outlook
1. File
2. Exit
3. Delete OST files from hard drive
1. Start
2. Search
3. For Files or folders
4. All files and Folders
5. Expand "More Advanced Options"
6. Check the box next to "Search hidden files and folders"
7. Leave the other options at default
8. In the top box for "All or part of the file name", type in
"*.ost"
9. Look in should be "Local Hard Drives"
10. Click Search
11. It'll find files such as outlook.ost, outlook001.ost, etc.
Select all the files it finds and select delete. This will
delete all the offline emails from your computer, but they
are still on the server so you won't lose anything.
4. Turn On Cache Mode
1. Open Outlook
2. Tools
3. Email Accounts
4. Select View or change existing e-mail accounts
5. Next
6. With "Microsoft Exchange Server" selected, Click "Change"
7. Check the box for "Use Cached Exchange Mode"
8. Next
9. In the box that pops up, click on OK
10. Click Finish
5. Restart Outlook
Once Outlook is restarted, all of your emails will download to your
computer again. It may take awhile, depending on the number of emails
you have. You can watch the progress in the lower right corner of your
Outlook screen… it will show the folders as they update.